Created in layers on a white wooden board surface--

First, a linear sketch was done in fine tip Sharpie.
Next, the blue background was added through the layering and blending of blue Sharpie with a solvent.
This layering process was repeated with various colors around the spheres, also with Sharpie.
Finally, tape was used to section off areas that would remain visible and several layers of black sharpie covered the rest.